In order to analyse the sensitivity of the equatorial ionospheric current system, comprising the solar quiet current system and the equatorial electrojet, to solar cycle variations and to the secular variation of the geomagnetic main field, we analyse 51 years (1935–1985) of geomagnetic observatory data from Huancayo (HUA), Peru, operated by Instituto Geofísico del Perú. This period is ideal to analyse the influence of the main field strength on the amplitude of the quiet daily variation, since the main field decreases significantly from 1935 to 1985, while the distance of the magnetic equator to the observatory remains stable. To this end, we digitise some 19 years of hourly mean values of the geomagnetic records of the , which have not been available digitally at the World Data Centres. The sensitivity of the amplitude of the quiet daily variation to both solar cycle variations (in terms of sunspot numbers and solar flux F10.7) and changes of the geomagnetic main field strength (due to secular variation) are determined. We confirm an increase of equatorial ionospheric current system for the decreasing main field in this period.